FIELD OF THE INVENTION
The invention relates to a brake system for a motor vehicle with electrically controlled wheel brake devices, which each comprise one control part and one actuator. The individual wheel brakes are activated by the devices independently of one another and in response to actuations of the brake pedal. There are provided two separate electrical control circuits, which are each provided for one portion of the wheel brake devices, both of which receive control signals from a device that detects actuations of the brake pedal and which each have their own power supply circuit, so that even in the event of a breakdown of one or more actuators, safe braking of the motor vehicle is ensured.
Virtually all passenger cars today are equipped with hydraulic brake systems. Increasing demand for additional brake functions--such as anti-lock brake systems, drive stabilizer systems, traction controls and sophisticated cruise controls--and demands to reduce the expenses for assembly and maintenance and to make the hydraulic devices smaller have led to the development of purely electrical brake systems (also known by the term "brake by wire").
In such electrical brake systems, the brake signal actuated by the driver is separated in terms of force from the brakes; that is, the brake moment demand on the part of the driver is no longer transmitted directly as force via a hydraulic (or compressed-air-actuated) system, but it is now transmitted only in the form of an electrical signal over electrical lines. With the electrical signal, the brake actuator (or brake actor) is controlled that generates the requisite brake moment with an independent energy supply. As a rule, this energy supply is furnished from the on-board electrical system, and the brake moment is generated by an electromechanical or electro-hydraulic actuator and it is transmitted to a brake disk or brake drum (see earlier German patent application DE 195 29 664.8, applicant's docket GR 95 P 1763).
Since the brake system is a central safety function of the motor vehicle, it must assure extensive freedom from failure, and in the event of a malfunction it must enable braking of the vehicle, at least with reduced action. To that end, a dual-circuit brake system is demanded by law; as a rule, it is realized in the form of two separate hydraulic brake circuits.
In a known brake system with electrically controlled wheel brake devices, which each comprise one control part and one actuator, by which the individual wheel brakes are activated independently of one another and as a function of actuations of the brake pedal, an emergency system is provided by which braking of the motor vehicle is enabled in the event that one or more actuators fails (European patent publication EP-A 0 486 281). Defective actuators are detected, and in the case of braking the functional actuators, or some of them, are actuated in accordance with a predetermined scheme.
Another known brake system has two separate electrical control circuits, each for two wheel brakes. Both of them receive control signals from a device that detects actuations of the brake pedal, and they each have their own energy supply circuit (British Patent GB 2 225 397 A). Additional brake functions that provide safety or comfort in motor vehicle operation cannot be achieved with this system.
In another known brake system, the control electronics are embodied in noncentralized fashion. A central module and a plurality of wheel modules each have microprocessors with their own "intelligence". The wheel modules are hierarchically subordinate to the central module (European patent publication EP 0 467 112 A). The expense for circuitry and programming is considerable.